[#789] blobovnicza: Implement iterator

There is a need to be able to process all stored objects saved in
`Blobovnicza`.

Implement `Blobovnicza.Iterate` method which iterates over all objects.
Implement `IterateObjects` helper function to simplify the code.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/local_object_storage/blobovnicza/blobovnicza_test.go

@@ -2,6 +2,7 @@ package blobovnicza
 
 import (
 	"crypto/sha256"
+	"encoding/binary"
 	"errors"
 	"math/rand"
 	"os"
@@ -9,6 +10,7 @@ import (
 
 	cidtest "github.com/nspcc-dev/neofs-api-go/pkg/container/id/test"
 	objectSDK "github.com/nspcc-dev/neofs-api-go/pkg/object"
+	objecttest "github.com/nspcc-dev/neofs-api-go/pkg/object/test"
 	"github.com/nspcc-dev/neofs-node/pkg/core/object"
 	"github.com/nspcc-dev/neofs-node/pkg/util/logger/test"
 	"github.com/stretchr/testify/require"
@@ -118,3 +120,57 @@ func TestBlobovnicza(t *testing.T) {
 
 	require.NoError(t, blz.Close())
 }
+
+func TestIterateObjects(t *testing.T) {
+	p := t.Name()
+
+	// create Blobovnicza instance
+	blz := New(
+		WithPath(p),
+		WithObjectSizeLimit(1<<10),
+		WithFullSizeLimit(100<<10),
+	)
+
+	defer os.Remove(p)
+
+	// open Blobovnicza
+	require.NoError(t, blz.Open())
+
+	// initialize Blobovnicza
+	require.NoError(t, blz.Init())
+
+	const objNum = 5
+
+	mObjs := make(map[string][]byte)
+
+	for i := uint64(0); i < objNum; i++ {
+		data := make([]byte, 8) // actual data doesn't really matter for test
+
+		binary.BigEndian.PutUint64(data, i)
+
+		mObjs[string(data)] = data
+	}
+
+	var putPrm PutPrm
+
+	for _, v := range mObjs {
+		putPrm.SetAddress(objecttest.Address())
+		putPrm.SetMarshaledObject(v)
+
+		_, err := blz.Put(&putPrm)
+		require.NoError(t, err)
+	}
+
+	err := IterateObjects(blz, func(data []byte) error {
+		v, ok := mObjs[string(data)]
+		require.True(t, ok)
+
+		require.Equal(t, v, data)
+
+		delete(mObjs, string(data))
+
+		return nil
+	})
+	require.NoError(t, err)
+	require.Empty(t, mObjs)
+}